Wireless inductive power transfer related technology employs near-field magnetic inductive coupling between an energy transmitter coil and an energy receiver coil to transfer energy through a high frequency (typically hundreds of kilo-Hertz or even mega-Hertz) magnetic field. The energy transmitter coil typically forms part of a transmitter and the energy receiver coil typically forms part of a receiver.
One important aspect of wireless power transfer is the accompanying data (information) communication between transmitter and receiver. Such information communication serves at least, but not limited to, one of the following functions:
a) localization of receivers on the surface of the transmitter (i.e. load and load-position detection);
b) compatibility checking of the receiver through an identification process (i.e. load identification);
c) configuring the transmitter or receiver based on the transferred information;
d) establishing a power transfer contract (a “power transfer contract” represents the parameters that characterize the power transfer);
e) exchanging power transfer status or error messages; and
f) monitoring of battery conditions.
The information communication can be bi-directional (from transmitter to receiver and from receiver to transmitter) or in a single direction. It can be implemented with existing communication methods, such as those used in RFID, NFC, Bluetooth, Wi-Fi, or others (Partovi, US 2007/0182367).
However, one disadvantage of such methods is that some communication IC and circuits need to be added to the transmitter and/or receiver for communication purposes, which introduces extra components, complexity and cost. In some instances, extra coils for data transfer are also required.
In most cases, data transfer from the receiver to the transmitter is more important and sometimes mandatory. One relatively simple method to achieve this purpose is by using a method called load modulation, in which some additional load impedance is switched on and off during communication so that the total load impedance is changed. For example, a receiver can include a resistor or a capacitor which is switched on and off for communication purposes. In particular, the changed load impedance influences some electrical characteristics of the transmitter so that the data can be detected and re-constructed.
Indeed, such load modulation methods have been widely used in RFID systems. Normally, however, the amplitude change of one electric parameter (like the voltage across the transmitter coil or the current through the coil) of the transmitter is detected and used for demodulation. This is called “amplitude modulation and demodulation”. It has been analyzed and shown that such amplitude demodulation is always valid because the load modulation is very “prominent” in RFID systems due to very little power being transferred.
However, in wireless power transfer, data transfer is accompanying energy transfer. Load modulation must be analyzed with the consideration of different loading conditions. In fact, there is a very wide range of loads having many different power requirements. This is totally different to RFID systems. Furthermore, when differences in coupling due to the different possible relative positions of the transmitter and receiver and the different distances between the transmitter and receiver are taken into account, load modulation becomes even more complex.
Moreover, the receiver may use many different methods to achieve load modulation. Besides the examples described above which use a resistor or a capacitor, some parameter in the receiver passive network (such a network can be a resonant tank, a filter or other functional network formed from passive components) or even the impedance of the receiver coil itself can be changed for the purpose of data transfer.
Thus, in developing a universal transmitter to work with different receivers using different load modulation methods, simple amplitude demodulation at the transmitter is not appropriate since it is not always valid. Therefore, there is a need for a universal demodulation method in order to develop a universal transmitter for standardized wireless power transfer.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.